The present invention relates to a gradiometer loop system of a device for the multichannel measurement of magnetic fields. Such a gradiometer loop system is known from European Pat. No. B-O 111 827.
For measuring very weak magnetic fields, the use of superconducting quantum interferometers, so-called "SQUIDs" (abbreviation for: Superconducting Quantum Interference Devices) is generally known. (See, for instance, "J. Phys. E.: Sci. Instr.", Vol. 13, 1980, pages 801 to 832 or "IEEE Trans. Electron Dev.", Vol. ED-27, No. 10, October 1980, pages 1896 to 1908). A preferred field of application for these interferometers is the field of medical diagnostics, magnetocardiography or magnetoencephalography, because the magnetic fields caused by magnetic heart or brainwaves have field strengths in the order of magnitude of only about 50 pT or 0.1 pT (see, for instance, "Biomagnetism-Proceedings Third International Workshop on Biomagnetism, Berlin 1980, Berlin/New York, 1981, pages 3 to 31). In addition, it should be possible to detect these very weak fields also in the presence of comparatively large interference fields. For measuring such biomagnetic fields in the order of magnitude mentioned, measuring devices are known which can be designed as single or, in particular multichannel devices. Generally, such a measuring device contains per channel a gradiometer of superconducting gradiometer loops, a SQUID as well as superconducting leads extending between the SQUID and the gradiometer loops. The loop facing the magnetic field source to be detected is designated also as the detection loop, while the loop spatially further removed therefrom can be considered as a compensation loop. With such a loop system the magnetic flux of the field source can advantageously be measured directly, and very high sensitivity and good discrimination against external interference fields can be achieved therewith.
The gradiometer loops as well as the corresponding connecting leads can be constructed, for instance, of superconducting wire, where the connecting leads can additionally be located in a superconducting shielding device. In general, however, only round loops can be made of wire with the required accuracy. In addition, the connecting leads cannot be contacted free of inductance and insensitive of fields. Furthermore, in the case of multichannel measuring devices, very many individual units must be handled. Instead of using discrete superconducting wires, the gradiometer loops and the corresponding connecting leads can also be deposited as thin-film conductors on carrier foils. The coated carrier foils are then applied to a carrier body with pronouncedly three-dimensional shape (see, for instance, European Pat. Application No. AO 185 186). In this embodiment, however, it cannot be avoided that the connecting leads forming pairs of parallel conductors are likewise field-sensitive due to their only finite conductor spacing.
In the multichannel measuring device known from the European Patent mentioned at the outset, the detection loops and the compensation loops are combined to form a planar field, a so-called array. The two loop arrays are arranged on two opposite flat sides of a three-dimensional carrier body. In this measuring device, however, the array area available for each array cannot be utilized completely for the gradiometer loops, because adjacent loops must be spaced sufficiently to bring the required connecting leads through. In this connection, it cannot be avoided that the connecting leads arranged there are field sensitive.